Definition
In computer networking, a frame is a unit of data transmission at the data link layer (Layer 2) of the OSI model. It encapsulates network-layer packets with headers and trailers that contain control information such as addressing, error detection, and synchronization data, enabling reliable point‑to‑point or point‑to‑multipoint communication over a physical medium.
Overview
Frames are the primary means by which devices on a local area network (LAN) or similar link-layer segment exchange information. When a higher‑layer protocol (e.g., IP) generates a packet, the data link layer packages that packet into a frame, adds its own addressing (e.g., MAC addresses), and appends error‑checking codes (commonly a cyclic redundancy check, CRC). The completed frame is then placed onto the physical medium (copper, fiber, wireless) where it is transmitted as a sequence of bits. Upon reception, the receiving interface validates the frame, extracts the payload, and forwards it to the appropriate upper‑layer protocol.
Different networking technologies define distinct frame formats. For example:
- Ethernet (IEEE 802.3) frames include a preamble, start‑frame delimiter, destination and source MAC addresses, a length/type field, payload, and a 32‑bit CRC.
- Wi‑Fi (IEEE 802.11) frames have similar fields but also contain control information specific to wireless medium access, such as sequence numbers and frame‑control flags.
- Point‑to‑Point Protocol (PPP) frames, used in serial links, contain a flag, address, control, protocol identifier, payload, and FCS.
Frames may also be subject to fragmentation when payload size exceeds the maximum transmission unit (MTU) of the underlying medium, although fragmentation is more commonly associated with packets at the network layer.
Etymology/Origin
The term “frame” derives from the metaphor of a structural frame that encloses or supports contents. In early data communication literature of the 1970s, “framing” described the process of delimiting a sequence of bits into recognizable units, analogous to framing a picture. Standardization efforts such as the IEEE 802 series formalized the term within the context of LAN technologies.
Characteristics
| Characteristic | Description |
|---|---|
| Layer | Data Link Layer (Layer 2) of the OSI model |
| Addressing | Typically uses MAC (Media Access Control) addresses; length varies by technology (e.g., 48‑bit for Ethernet) |
| Control Information | Includes start/end delimiters, length/type fields, sequence numbers, and flags for flow control |
| Error Detection | Commonly a CRC or checksum placed in a trailer; some technologies also support error correction |
| Size | Minimum and maximum frame sizes are defined per technology (e.g., Ethernet: 64 bytes minimum, 1518 bytes standard maximum) |
| Encapsulation | Carries network‑layer packets (e.g., IPv4, IPv6) as payload |
| Transmission | Sent as a contiguous bit stream synchronized to the physical layer’s signaling (e.g., NRZ, Manchester coding) |
| Reliability | Reliability mechanisms (e.g., acknowledgments, retransmission) are generally implemented in higher layers; the data link layer focuses on local error detection and possibly correction |
Related Topics
- Packet (networking) – The unit of data at the network layer (Layer 3) that is encapsulated within a frame.
- MAC address – The hardware identifier used in most frame headers for source and destination addressing.
- Cyclic Redundancy Check (CRC) – A common error‑detecting code appended to frames.
- Ethernet – A widely deployed LAN technology defining a specific frame format.
- IEEE 802.11 (Wi‑Fi) – Wireless LAN standard with its own frame structure.
- Maximum Transmission Unit (MTU) – The largest payload size that can be contained in a single frame without fragmentation.
- Data Link Layer Protocols – Protocols such as LLC (Logical Link Control) and SNAP (Subnetwork Access Protocol) that operate within the frame’s payload.
This entry reflects current, verifiable knowledge about the networking concept of a frame as of the latest standards and publications.